The object of the invention is the thermal insulation of a production column in marine applications at great depth. The column (duct) extends substantially vertically between the surface of the sea and a bed located for example 1500 meters under said surface.
The temperature of the sea water at the bed can be very cold, of a few degrees, and for example less than 5° C. The temperature of the sea water at the surface can be between 10° C. and 20° C. The temperature at the bed is very low, and there is a temperature gradient of the sea water between the bed and the surface.
The hydrostatic pressure at the sea bed is very substantial, as it is linked to the substantial depth of the application under consideration which is between 500 meters and 2000 meters, and possibly up to 3000 meters deep.
The temperature of the fluid or hydrocarbon effluent depends on the subsea well. At these substantial hydrostatic pressures, if the temperature of the fluid is lowered under a limit temperature, there is a great risk of provoking the formation of hydrates, or of paraffin. These solid compounds can clog the columns or ducts.
The objective is to reduce as much as possible the variation (reduction) in the temperature of the fluid between its temperature at the sea bed and at the surface in such riser columns or ducts. The applications can be as production of hydrocarbon fluid or a system for recovering hydrocarbon fluid in the case of leaks on such wells at great depths.
In order to retain a substantial temperature of the fluid during its rising, if possible close to the outlet temperature at the sea bed, it is useful to have a thermal insulation for this column.
As such, the formation of hydrates in the column can be prevented. Furthermore, the higher the temperature of the fluid arriving at the surface is, the more transporting it and treating it will be facilitated on the installations at the surface, such as well testing installations. The treatment facility at the surface makes it possible for example to focus on regulating a recovery or confinement system at the sea bed, a system which is delicate to implement, as was shown by the experience with the incident on the “Deepwater Horizon” platform in the Gulf of Mexico.
There are two known solutions for the thermal insulation of a riser column. The production column on wells at great depths can be deployed according to two modes:                with a drilling marine riser, with this riser or tube surrounding the column over the entire height between the surface and the sea bed, it protects the production column from mechanical stresses, or        without a drilling marine riser, therefore by using a drilling or production column, directly in the open sea, this solution is referred to as “Open Sea”.        
In the first case (with a drilling marine riser), there is an annular space between the riser and the column, and the thermal insulation of the production column can be carried out by the following methods:                a) The annular space can be filled with a fluid that can be circulated using a pump and heated using a heat exchanger on the surface;        b) The annular space can be filled with an insulator gel;        c) The production column can be carried out using a tube and the “pipe-in-pipe” technique, with this tube having mechanical characteristics (resistance to compression of the tube load) that cannot be used directly in the depth of the water directly in the open sea;        d) The production column can be provided with electric tracing in order to heat with the Joule effect by passing an electric current in said tracing.        
This first solution has the disadvantage of needing to use a device referred to as a blow out preventer (BOP), or at least a device referred to as a lower mariner riser package (LMRP). This equipment is heavy and generates on the fixed installations on the sea bed, substantial mechanical stresses (weight, tension, moment) which leads to over-sizing this equipment.
In the second case (without a drilling marine riser, i.e. using the production column in the open sea), the thermal insulation of the production column can be carried out by a method of setting up a solid outside insulator around the production column.
This solution has the disadvantage of needing to use a column of a specific type, for example having fastening bodies and for example of the “quick union” type. They also have the disadvantage of requiring the use of means of bracing, of lifting and of testing at a pressure suited to these fastening bodies. These means are heavier and more cumbersome and usually are not present on drilling machines. These columns are heavy and their implementation time is long. However, a heavy column does not make it possible to reach great depths, and in particular depths exceeding 1000 meters. Furthermore, in order for such an insulator added to a production column to resist the hydrostatic pressure of the environment at the sea bed, such an insulator is very cumbersome (large diameter around the production column). Furthermore, the production column is then very difficult to disassemble, especially if the insulator is superimposed on the joints between the succession of sections of the column, or then it can be disassembled in a long period of time.